Two new genera and four new species of Asterocheridae (Copepoda: Siphonostomatoida) associated with sponges (Porifera) from the Korean East Sea

Two new genera and four new species of siphonostomatoid copepods of the family Asterocheridae associated with sponges are described from the Korean East Sea (Sea of Japan). These new copepods are distinguishable from related genera and species by their diagnostic morphological characters as follows: Amalomyzon elongatum n. gen. n. sp. bears an elongated body, two-segmented rami of legs 2, a uniramous leg 3 with two-segmented exopod, and a rudimentary leg 4 represented by a lobe. Dokdocheres rotundus n. gen. n. sp. has an 18-segmented female antennule, a two-segmented endopod of antenna, and unusual setations of swimming legs, including three spines plus four setae on the third exopodal segment of legs 2–4. Asterocheres banderaae n. sp. has no inner coxal seta on leg 1 or 4, but has two strong, sexually dimorphic inner spines on the second endopodal segment of male leg 3. Scottocheres nesobius n. sp. bears elongate female caudal rami about six times longer than wide, a 17-segmented female antennule, and two spines plus four setae on the third exopodal segment of leg 1.


INTRODUCTION
Siphonostomatoid copepods of the family Asterocheridae live in association with marine invertebrates. The most commonly reported hosts of these copepods are sponges, cnidarians, and echinoderms, although hosts are unknown for many species of copepods (Boxshall & Halsey, 2004). The great majority of the copepods of this family have been recorded from European and tropical seas with very few recorded in the entire Pacific region. Ho (1984) was the first one who recorded a species of the Asterocheridae in the Korean East Sea (Sea of Japan), who described Asterocheres aesthetes as a new species associated with sponge Spirastrella insignis Thiele, 1898 in Sado Island, Japan. Since then, seven additional species have been reported from the same sea (three associated with sponges, two with sea stars, and two with unknown hosts) ( Table 1).

MATERIALS & METHODS
The copepods studied in the present work were extracted from sponges collected at Ulleung Island and Dokdo Island, the most remote islands of Korea located in the Korean East Sea (Sea of Japan) (Fig. 1). Sponge hosts were collected by trimix and SCUBA diving at depths of 22 m to 45.2 m. Copepods extracted from these sponge hosts were preserved in 80% ethanol. Permits for marine organisms collection and export were given by the county office of Ulleung-gun, Korea. For microscopic observation, copepods were immersed in lactic acid for at least 10 min and dissected. Dissected appendages were observed using the reverse slide method of Humes & Gooding (1964). Drawings were made under a light microscope equipped with a drawing tube. In the armature formula for the description of species, Roman numerals indicate spines and Arabic numerals represent setae. Lengths of copepod specimens and measurements of appendages were mostly based on a dissected and figured specimen of each species. Morphological terminology followed Huys & Boxshall (1991). Type specimens have been deposited in the National Institute of Biological Resources (NIBR), Incheon, Korea.
This published work and nomenclatural acts it contains have been registered in ZooBank, an online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information can be viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: [urn:lsid:zoobank.org:pub:0E84F5CE-D4C5-4AAD-A72C-CCBD77806CDB]. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central SCIE and CLOCKSS.  Diagnosis. Body elongate, cylindrical, incompletely segmented. Urosome foursegmented; caudal ramus with six setae. Female antennule 20-segmented, with large aesthetasc on antepenultimate segment; first segment with one seta. Male antennule 18-segmented, geniculate between 15th and 16th segments and between 16th and 17th segments. Antenna consisting of coxa, basis, one-segmented small exopod, and threesegmented endopod; third endopodal segment terminating in large spine. Oral siphon extending over insertions of maxillipeds. Mandible consisting of thin stylet and indistinctly two-segmented palp tipped with two subequal setae. Maxillule bilobed; inner lobe about 3 times longer than outer lobe, tipped with four large setae; outer lobe tipped with three setae. Maxilla two-segmented, distal segment claw-like. Maxilliped consisting of syncoxa, basis, and four-segmented endopod. Leg 1 with three-segmented rami. Leg 2 with two-segmented rami. Leg 3 with obscurely segmented protopod, two-segmented exopod, lacking endopod. Leg 4 as vestigial lobe bearing two setae. Armature formula for legs 1-3 as follows: leg 5 represented by one seta on fifth pedigerous somite and one-segmented exopod bearing three setae.
Legs 1-3 as in female. Leg 4 ( Fig. 4D) incompletely two-segmented; proximal segment with one seta laterally; distal segment unarmed or armed with one apical seta. Leg 5 ( Etymology. The specific name refers to the elongate body of the new species. Remarks. The body form of Amalomyzon elongatum n. sp. is similar to that of Tuphacheres micropus Stock, 1965 known in the Mediterranean Sea (Stock, 1965). However, these two species differ from each other at the generic level as they exhibit different ways of leg segmentation reductions. Dokdocheres n. gen. urn:lsid:zoobank.org:act:6787FECB-E7FA-48DB-97CF-73EBDEB4E151 Diagnosis. Body broad, dorsoventrally flattened. Prosome consisting of cephalothorax and three metasomal somites. Urosome four-segmented, with two-segmented abdomen. Caudal ramus with six setae. Antennule 18-segmented, with aesthetasc on antepenultimate segment; first segment with one seta. Antenna consisting of coxa, basis, one-segmented exopod, and 2-segmented endopod; exopodal segment elongate, more than half length of first endopodal segment, with three setae; second endopodal segment terminating in slender claw. Oral cone short, stout. Mandible consisting of stylet bearing six teeth distally and palp tipped with one seta. Maxillule bilobed; inner lobe tipped with three large setae; outer lobe with three large and one small setae. Maxilla 2-segmented; distal segment as claw.
Etymology. The name of the new genus is derived from Dokdo Island, the type locality of the type species, and -cheres, the ending of Asterocheres, the type genus of the family Asterocheridae. Gender masculine.
Remarks. The establishment of Dokdocheres n. gen. is justifiable by multiple extraordinary features in the leg setation of its type species, D. rotundus n. gen. n. sp., as follows: (1) the third exopodal segment of leg 1 is armed with 1 spine plus five setae (armature formula I+1, 2, 2), which is a unique armature condition in the family Asterocheridae; (2) the armature formula of the third endopodal segment of leg 1 is 1, 1+I, 3 (having 1 distal spine), which is a feature shared by three genera (Gomumucheres Humes, 1996, Parasterocheres Humes, 1996, and Phyllocheres Humes, 1996 within the family, all of which are associated with the sponge Dysidea in the Moluccas (Humes, 1996); (3) the armature formula of the third exopodal segment of legs 2-4 is II, I, 4. The same armature formula of the third exopodal segment is exhibited on leg 2 in 15 genera, leg 3 in 10 genera, and leg 4 in four genera. However, only Dokdocheres n. gen. and two existing genera (Siphonopontius Malt, 1991 andStenomyzon Kim, 2010) exhibit the same armature condition in all legs 2-4; (4) the armature formula of the third endopodal segment of leg 4 is 0, I, 2. This armature condition is shared by five existing genera (Asterocheroides Malt, 1991, Cecidomyzon Stock, 1981, Cephalocheres Kim, 2010, Hammatimyzon Stock, 1981, and Scottomyzon Giesbrecht, 1897. Therefore, Dokdocheres n. gen. can be recognized by the above first feature alone or by the combination of the other three unusual features. monocuspid outer distal process. Third exopodal segment of leg 1 with characteristic armature, with one spine plus five setae. Armature formula for legs 1-4 as follows: Leg 5 (Figs. 5B and 7C) two-segmented, consisting of protopod and one-segmented exopod; protopod, wider than long, not articulated from somite, with one long, naked seta (59 µm long) dorsodistally; exopodal segment 28 × 12 µm, with three weakly pinnate setae
Oral cone, mandible, maxillule, and maxilla as in female. Maxilliped (Fig. 10D) segmented as in female, but inner subdistal element on syncoxa transformed to spine tipped on protrusion of syncoxa; second endopodal segment bearing one small seta.
Legs 1, 2, and 4 as in female. Leg 3 endopod (Fig. 10E) sexually dimorphic: second segment with 2 massive, spinulose spines (instead of setae in female); distal spine on third segment longer than that of female. Leg 5 as in female. Leg 6 ( Fig. 10B) represented by two unequal setae on genital operculum.
Etymology. The new species is named in honor of Dr. Bandera for her contribution to the taxonomy of the genus Asterocheres.
Remarks. The genus Asterocheres is inconveniently large, comprising about 90 known species (WoRMS Editorial Board, 2022), many of which are incompletely described. Nevertheless, A. banderaae n. sp. is distinguishable from its congeners by its several characteristic morphological features. The coxa of leg 1 of the new species lacks an inner seta. According to Bandera & Conradi (2016), this feature is also present in three existing species of Asterocheres (A. eugenioi Bandera & Conradi, 2014, A. sarsi Bandera and Conradi, 2009, and A. trisetatus Kim, 2010. In A. eugenioi and A. sarsi, caudal rami are 1.5 or more times longer than wide with the oral cone extending to the insertion of leg 1 (Bandera & Conradi, 2014). In A. trisetatus, the third endopodal segment of leg 1 is elongated (Kim, 2010). These features of the three species are not applicable to A. banderaae n. sp.
The second endopodal segment of leg 3 is sexually dimorphic, armed with two pinnate setae in the female, but with two strong specialized spines in males. Sexual dimorphism in leg 3 has been reported in several species of Asterocheres, such as A. bahamensis Kim, 2010, A. nidorelliae Reyes-González & Suárez-Morales, 2021, A. peniculatus Kim, 2010, A. plumosus Kim, 2010, A. urabensis Kim, 2004, and A. walteri Kim, 2004(Kim, 2004Kim, 2010;Reyes-González & Suárez-Morales, 2021). However, in all these species, sexual dimorphism in leg 3 occurs on the third endopodal segment. None of them occurs on the second endopodal segment as in A. banderaae n. sp. They also differ from the new species in various other morphological aspects.
The syncoxa (first segment) of the male maxilliped of the new species bears a specialized inner distal spine. This feature is shared only with A. cuspis Kim, 2016 known in the eastern coast of Korea. Interestingly, like A. banderaae n. sp., A. cuspis has a posteriorly tapering body and a proximally swollen inner distal seta on the basis of leg 1. Despite these similarities, A. cuspis is distinguished from the new species by having a two-segmented mandibular palp (cf. 1-segmented in A. banderae n. sp.) and an inner coxal seta on leg 1 without sexual dimorphism on the second endopodal segment of leg 3.
Oral siphon, mandible, maxillule, and maxilla as in female. Maxilliped (Fig. 12H) slightly different from that of female in having angular protrusion at proximal quarter of inner margin of basis. Legs 1-5 as in female. Leg 6 represented by 3 small setae on genital operculum (Fig. 12F).
Etymology. The specific name nesobius is derived from Greeks neso (= island) and bio (= life), alluding to the discovery of the species in the island.
Remarks. The third exopodal segment of legs 2-4 of Scottocheres nesobius n. sp. bears two outer spines (armature formula II, I, 4 in legs 2 and 3, and II, I, 3 in leg 4). In the genus Scottocheres, these armature conditions are shared only with S. laubieri Stock, 1967 and S. mipoensis Kim, 2016. Otherwise, these two species distinctly differ from the new species as they have short caudal rami which are as long as wide or wider than long (Stock, 1967;Kim, 2016) with the third exopodal segment of leg 1 armed with three spines plus four setae (formula III, 2, 2) rather than two spines plus four setae (armature formula II, 2, 2) as in the new species.
Regarding the dimension of the caudal ramus, S. nesobius n. sp. appears to be similar to S. gracilis Hansen, 1923 as the latter species has elongated caudal rami which are five times longer than wide (Hansen, 1923). Scottocheres gracilis, a deep-sea species known from the southwest of Iceland, was obscurely described, with unknown leg armature. In S. gracilis, according to Hansen (1923), the caudal ramus is slightly shorter than the first and second free abdominal somites combined (''second and third abdominal somites combined'' according to Hansen) (cf. distinctly longer in the new species). The body of the female is 1.05 mm long, which is significantly longer than that (about 0.7 mm long) of the new species. The exopodal segment of leg 5 extends beyond the tooth-like lateral process of the genital double-somite (cf. the exopodal segment terminates before the process in the new species).

CONCLUSIONS
Two new genera and four new species of copepods of the family Asterocheridae associated with sponges are described from the Korean East Sea (Sea of Japan). Hosts and differential characters of the new copepod species are as follows: Amalomyzon elongatum n. gen. n. sp. is associated with Dysidea dokdoensis Kang, Lee & Sim, 2020. It bears an elongated body, two-segmented rami of legs 2, uniramous leg 3 with a two-segmented exopod, and rudimentary leg 4 represented by a lobe.
Dokdocheres rotundus n. gen. n. sp. is associated with Acanthella vulgata Thiele, 1898. It has an 18-segmented female antennule, 2-segmented endopod of the antenna, and unusual setations of swimming legs including the presence of three spines plus four setae on the third exopodal segment of legs 2-4.
Asterocheres banderaae n. sp. is associated with Petrosia corticate (Wilson, 1925). It bears no inner coxal seta on leg 1 or 4, but has two strong, sexually dimorphic inner spines on the second endopodal segment of male leg 3.
Scottocheres nesobius n. sp. is associated with Myxilla producta Hoshino, 1981. It is characterized by elongate female caudal rami about six times longer than wide, a 17segmented female antennule, and an armature of two spines plus four setae on the third exopodal segment of leg 1.
After adding the two new genera and four new species described here, the family Asterocheridae in the Korean East Sea now comprises 12 species in seven genera.